Understanding the major features and subtleties of indoor exposures to particles of outdoor origin can improve our understanding of the exposureâ€"response relationship on which ambient air pollutant standards are based. The EPA's Indoor Environments Division commissioned the National Academies of Sciences, Engineering, and Medicine to hold a workshop examining the issue of indoor exposure to PM more comprehensively and considering both the health risks and possible intervention strategies. Participants discussed the ailments that are most affected by particulate matter and the attributes of the exposures that are of greatest concern, exposure modifiers, vulnerable populations, exposure assessment, risk management, and gaps in the science. This report summarizes the presentations and discussions from the workshop.